1. Field of the Invention
This invention relates to the separation of closely boiling organic compounds. More particularly, this invention relates to the separation of ethylene glycol and N-methylpyrrolidone. This invention is especially concerned with recovering ethylene glycol as a product by means of azeotropic distillation from reaction mixtures employing N-methylpyrrolidone as a solvent.
In one method of producing ethylene glycol, disclosed in commonly assigned U.S. patent application Ser. No. 884,877, filed Mar. 9, 1978, synthesis gas is employed to convert formaldehyde to ethylene glycol in the presence of a rhodium catalyst. N-methylpyrrolidone is a preferred solvent in this process. Recovery of the N-methylpyrrolidone solvent for re-use and separation of pure ethylene glycol from the reaction mixture constitute important aspects in the commercial attractiveness of the overall process. However, the relative volatilities and the boiling points of the solvent and the desired product are so close that fractional distillation in commercial distillation equipment is impractical and uneconomical.
A process which would permit the effective and economic recovery of ethylene glycol from the N-methylpyrrolidone solvent is highly desirable.
2. Description of the Prior Art
Azeotropic distillation is a well known means of separating two different compounds or types of compounds having boiling points in close proximity. A third component which forms an azeotrope with one of the closely boiling components is added, the mixture is subjected to distillation and the azeotrope is removed as the overhead thereby effecting separation of the closely boiling components. This third component, called an azeotroping agent, is often separated subsequently from the component with which it forms the azeotrope by conventionally known means, such as decanting, and returned to the distillation apparatus for reuse. Each closely boiling binary system presents its own special problems so as to render past experience of little value and future results unpredictable. Thus, the selection of an azeotroping agent is seldom a simple task. Not only must the azeotroping agent form an azeotrope with the right volatility but the components of the azeotrope must be capable of being separated subsequently in highly pure form for either reuse in the process or recovery as a final saleable, useful product. In addition, the azeotroping agent should be relatively inexpensive, non-toxic, non-reactive and non-corrosive.
Ethylene glycol has been the subject of azeotropic distillation in the prior art. U.S. Pat. No. 2,218,234 discloses the use of toluene to form a three component azeotrope with ethylene glycol and water thereby effecting a separation of these components from a mixture occurring in the photographic industry which also contains salts, dyes and other contaminants. Aromatic hydrocarbons, other than benzene, i.e. toluene, ethyl benzene, etc., have been found in U.S. Pat. No. 3,074,962 to be effective in separating ethylene carbonate from ethylene glycol by forming an azeotrope with the glycol. In U.S. Pat. No. 3,809,724, mixtures of ethylene glycol and its lower carboxylate esters have been separated by employing as an azeotroping agent with the ethylene glycol, organic compounds having a boiling point between 135.degree.-190.degree. C. such as, acyclic and cyclic saturated hydrocarbons, alkyl-substituted benzenes and halogenated aromatic hydrocarbons. U.S. Pat. No. 4,021,311 is closely related to U.S. Pat. No. 3,809,724 and discloses that 1,2,3-trimethyl benzene is a particularly effective azeotroping agent in separating ethylene glycol from its carboxylate esters.
Ethylene glycol itself has found utility as an azeotroping agent. U.S. Pat. No. 3,392,090 discloses that ethylene glycol will form an azeotrope with less polar alkyl phenols to effect a separation from more polar alkyl phenols.
N-Methylpyrrolidone has been disclosed in U.S. Pat. No. 3,132,078 as forming an azeotrope with non-naphthalenic hydrocarbons to produce the overhead while naphthalenic hydrocarbons pass into the bottoms product.
The prior art provides few guidelines to the skilled artisan faced with the task of separating ethylene glycol from N-methylpyrrolidone. Both have functioned as azeotropic agents while ethylene glycol has been recovered as a part of an azeotrope from a variety of mixtures but not, so far as the prior art teaches, from a mixture comprising N-methylpyrrolidone.
It is thus among the objects of this invention to provide a means for effectively separating a mixture comprising ethylene glycol and N-methylpyrrolidone.